1. Technical Field
The present disclosure relates to a complementary metal-oxide semiconductor (CMOS) image sensor and a method of driving the same, and more particularly, to a CMOS image sensor and a method of driving the same that can reduce the number of devices required by each of a plurality of pixels and can stably drive the pixels.
2. Discussion of Related Art
In general, image sensors are classified into charge coupled device (CCD) image sensors or complementary metal-oxide semiconductor (CMOS) image sensors. CCD image sensors comprise a photocarrier accumulation unit that photographs an external object, absorbs light, and accumulates photocarriers, a transmission unit that transmits the accumulated photocarriers, and an output unit that outputs the photocarriers transmitted by the transmission unit as electrical signals.
Photodiodes are generally used as photocarrier accumulation units in the CCD image sensors. Photocarriers accumulated in a photodiode are transmitted externally by a transmission unit and an output unit of the CCD image sensor. Once the detection of an electrical signal by the CCD image sensor is terminated, the photocarriers accumulated in the photodiode must be discharged to make way for a subsequent image sensing operation. This operation is referred to as a reset operation.
It is more complicated to drive CCD image sensors, which operate as described above, than to drive CMOS image sensors, and CCD image sensors consumer more power than CMOS image sensors. Therefore, CMOS image sensors, which consume less power and offer a higher integration density than CCD image sensors, are more widely used.
FIG. 1A is a circuit diagram of a pixel 10 of a typical CMOS image sensor. Referring to FIG. 1A, the pixel 10 includes a photodiode PD that generates photocarriers by receiving light, and a plurality of transistors, including a transfer transistor T1, a resent transistor T2, a drive transistor T3, and a select transistor T4.
The transfer transistor T1 transmits photocarriers accumulated in the photodiode PD to a floating diffusion region FD in response to a transmission control signal Tx. The reset transistor T2 resets the electric potential of the floating diffusion region FD to a power supply voltage VDD in response to a reset signal Rx, thereby discharging photocarriers stored in the floating diffusion region FD.
The drive transistor T3 serves as a source follower-buffer amplifier. The select transistor T4 performs an addressing operation. More specifically, the select transistor T4 is turned on in response to a selection control signal Sx and, thus, transmits an output signal of the pixel 10 to an external device via an output port OUT. A load transistor T5 may be connected to the pixel 10. The load transistor T5 can read the voltage of the output signal of the pixel 10 under control of a predetermined load control signal LOAD.
The operation of the pixel 10 will now be described in detail with reference to FIG. 1B.
Referring to FIG. 1B, the selection control signal Sx is activated and input to selected ones of a plurality of pixels from which data is to be read, including the pixel 10. Then, the select transistor T4 is turned on, and the pixel 10 is chosen.
During the activation of the reset signal Rx, the reset transistor T2 is turned on, and the floating diffusion region FD is reset. Then the reset signal Rx is inactivated, and the voltage of a signal output from the output port OUT is detected at a time a.
Thereafter, when the transmission control signal Tx is activated, photocarriers accumulated in the photodiode PD are transmitted to the floating diffusion region FD. Then, the voltage of the floating diffusion region FD decreases. During the activation of the transmission control signal Tx, sufficient photocarriers are transmitted to the floating diffusion region FD. Thereafter, the transmission control signal Tx is inactivated, and the voltage of the signal output from the output port OUT is detected at a time b. The difference between the voltage detected at the time a and the voltage detected at the time b is calculated, and analog data obtained using the calculation is converted into digital data. In this manner, it is possible to obtain information regarding the amount of light sensed by the pixel 10.
FIG. 2A is a circuit diagram of a pixel 20 of a conventional CMOS image sensor having a tri-transistor structure. The number of transistors required by each pixel of the CMOS image sensor illustrated in FIG. 2A is much smaller than the number of transistors required by each pixel of the CMOS image sensor illustrated in FIG. 1A.
FIG. 2B is a diagram illustrating waveforms of control signals used to drive the pixel 20 of the CMOS image sensor illustrated in FIG. 2A. The operation of the CMOS image sensor illustrated in FIG. 2A will now be described in detail with reference to FIGS. 2A and 2B.
Referring to FIG. 2A, the pixel 20, which has a tri-transistor structure, includes a photodiode PD that generates photocarriers, and a plurality of transistors, including a transfer transistor T5, a reset transistor T6, and a drive transistor T7.
The transfer transistor T5 transmits photocarriers accumulated in the photodiode PD to a floating diffusion region FD in response to a transmission control signal TRF. The reset transistor T6 resets the electric potential of the floating diffusion region FD to a power supply voltage VDD in response to a reset signal RST. The drive transistor T7 serves as a source follower-buffer amplifier. The pixel 20, unlike the pixel 10 illustrated in FIG. 1A, does not require a select transistor.
Referring to FIG. 2B, a voltage signal DRN having a logic high level is input to an electrode of the drive transistor T7. When the reset signal RST is activated, the reset transistor T6 is turned on and resets the electric potential of the floating diffusion region FD to the level of the voltage signal DRN. Thereafter, the reset signal Rx is inactivated, and the voltage of a signal output from the output port OUT is detected a time a.
When the transmission control signal TRF is activated, the photocarriers accumulated in the photodiode PD are transmitted to the floating diffusion region FD. Thereafter, when the transmission control signal TRF is inactivated, the voltage of the signal output from the output port OUT is detected at a time b.
When a sensing operation of the pixel 20 is terminated, it is necessary to deselect the pixel 20. To accomplish this, the voltage signal DRN becomes logic low, and the reset signal RST is activated for a predetermined time period. Then, the voltage of the floating diffusion region FD decreases, and the drive transistor T7, whose gate is connected to the floating diffusion region FD, is turned off, thereby deselecting the pixel 20.
In the case of a conventional CMOS image sensor comprised of a plurality of pixels having a tri-transistor structure, it is necessary to alter the level of a voltage signal applied to an electrode of a drive transistor T7. Noise, however, may be generated while a switching operation is performed to change the level of the voltage signal. Once noise is generated in the voltage signal input to the drive transistor T7, unstable data is likely to be output due to variations in the gain of the drive transistor T7.